Powdery Mildew Resistance Genes in Wheat: Identification and Genetic Analysis

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www.ccsenet.org/jmbr Journal of Molecular Biology Research Vol. 1, No. 1; December 2011

ISSN 1925-430X E-ISSN 1925-431820

Powdery Mildew Resistance Genes in Wheat: Identification and

Genetic Analysis

Md. Ashraful Alam

College of Agronomy, Northwest A & F University, China

&

Scientific Officer, Wheat Research Centre, BARI, Dinajpur, Bangladesh

E-mail: [email protected]

Fei Xue


Changyou WangCollege of Agronomy, Northwest A & F University, China

Wanquan Ji (Corresponding author)


E-mail: [email protected]

Received: August 12, 2011 Accepted: August 25, 2011 Published: December 31, 2011

doi:10.5539/jmbr.v1n1p20 URL: http://dx.doi.org/10.5539/jmbr.v1n1p20

Abstract

Wheat powdery mildew, caused by Blumeria graminis f. sp. Tritici is one of the most devastating diseases of

common wheat worldwide. To date, 41 loci (Pm1 to Pm45, Pm18=Pm1c, Pm22=Pm1e, Pm23=Pm4c,

Pm31=Pm21) with more than 60 genes/alleles for resistance to powdery mildew have been identified and located

on 18 different chromosomes in bread wheat. 29 resistance genes/alleles have been tagged with molecular

markers such as restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNAs

(RAPDs), amplified fragment length polymorphisms (AFLPs), sequence tagged sites (STS) and simple sequence

repeats (SSRs), by using F2, back-cross populations, near-isogenic lines (NILs), doubled haploids (DH),

recombinant inbred lines (RILs) or bulked segregant analysis (BSA). The detail information on chromosomal

location, molecular markers linked to powdery mildew, mapping population and molecular mapping of powdery

mildew resistance genes have been reviewed.

Keywords:Blumeria graminis f.sp. tritici, Molecular markers, Mapping population, Molecular mapping,Powdery mildew resistance gene, Triticum aestivum

1. Introduction

Wheat powdery mildew, caused by Blumeria graminis (DC.) E.O. Speer f. sp. Tritici Em. Marchal (Bgt) =

Erysiphe graminis DC. Ex Meratf. sp. Tritici Em. Marchal, is one of the most devastating diseases of common

wheat occurs in many areas, including China, Germany, Japan, Russia, United Kingdom, South and West Asia,

North and East Africa, and the Southeastern United States (Bennett, 1984). Yield losses ranging from 13 to 34%

due to this disease (Griffey et al., 1993; Leath & Bowen, 1989). Growing of resistant cultivars offer effective,

economically sound and environmentally safe approach to eliminate the use of fungicides and reducing crop

losses caused by powdery mildew. There are two types of resistance to powdery mildew. One is called

monogenic (vertical) or race specific resistance, which is effective for some isolates of powdery mildew, but

ineffective for others. Race specific resistance is mainly via a hypersensitive foliar reaction directly involving

single major R genes, designated asPm (powdery mildew) genes, in a gene-for-gene interaction (Bennett, 1984;


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Chen and Chelkowski, 1999; Hsam and Zeller, 2002). Race-specific resistance genes are expressed in seedlings

and throughout the vegetative cycle of wheat. Though race-specific resistance have been extensively used in

wheat breeding programs, selection pressure exerted by cultivars with race-specific resistance genes results in

the rapid build-up of isolates with matching virulence genes. Afterward, race-specific resistance breaks down

when confronted by pathogen isolates with matching virulence genes and, therefore, is ephemeral.

Another type of resistance to powdery mildew is called adult plant resistance (APR), which retards infection,

growth and reproduction of the pathogen in adult plants but not in seedlings. It is also called slow mildewing

(Shaner, 1973) and partial resistance (Hautea et al., 1987). This type of resistance can be identified in cultivars

with defeated race-specific genes or lacking known race-specific resistance genes. APR to powdery mildew is

more durable than race-specific resistance. For example, APR in wheat cultivar Knox and its derivatives

remained effective against powdery mildew infection during the 20 years in which these cultivars were grown

commercially (Shaner, 1973). Massey, a derivative of Knox62, was released from Virginia Tech in 1981 (Starling

et al. 1984), and still has effective powdery mildew resistance in adult plants. Up to now, 41 loci (Pm1 to Pm45,

Pm18=Pm1c, Pm22=Pm1e, Pm23=Pm4c, Pm31=Pm21) with more than 60 genes/alleles for resistance to

powdery mildew have been identified and located on various chromosomes in bread wheat and its relatives (Ma

et al., 2011; McIntosh et al., 2008; Luo et al., 2009; Li et al., 2009; Hua et al., 2009; He et al., 2009). However,

resistances of genes are frequently overcome by new Bgtisolates, because the presence and frequency of virulence

genes in the pathogen population changes continuously (Leath & Murphy, 1985; Menzies & MacNeil, 1986; Limpert

et al. 1987; Namuco et al. 1987). The effective management strategy has been to replace cultivars when theirresistance is no longer effective (Wolf, 1984; Leath & Heun, 1990).

Molecular markers are now widely used for gene tagging, gene mapping, and other genetics research because they

are not influenced by environmental conditions and growth stage. The use of PCR based molecular markers to tag

genomic regions are more efficient for marker assisted selection (MAS), due to the small amount of DNA

template required and easy to handling of large population sizes. The identification of molecular markers of

flanking disease resistance genes, simplifies breeding activities such as cultivar development (Bonnett et al.,

2005), near isogenic line development (Zhou et al., 2005), and pyramiding resistance genes into single genotypes

by marker assisted selection (MAS). Many of the recently reported ofPm genes have associated markers

(Miranda et al., 2006, 2007; Perugini et al., 2008). Recently, molecular markers such as restriction fragment

length polymorphisms (RFLPs), random amplified polymorphic DNAs (RAPDs), amplified fragment length

polymorphisms (AFLPs), sequence tagged sites (STS) and microsatellites, also termed simple sequence repeats

(SSRs), have been widely used to tag and identify powdery mildew resistance genes in wheat by using F2 andback-cross populations, near-isogenic lines (NILs), doubled haploids (DH), recombinant inbred lines (RILs) or

bulked segregant analysis (BSA, Michelmore et al., 1991).

2. Chromosomal Location of IdentifiedPmGenes

In recent years, wheat genomics research has increased the use of genetic maps to position a gene of interest between

close flanking markers (Haley & Knott, 1992). The application of molecular markers in plant systems increases the

efficiency of conventional plant breeding by carrying out indirect selection through molecular markers linked to the

traits of interest (Gupta et al., 1999). A linkage map gives information on the position of markers within a linkage

group. The map positions are inferred from estimates of recombination frequencies between markers. The distance

between these markers is expressed in centimorgan (cM) which represents the recombination rates between them

(Jones et al., 1997). Chromosomal positions of several mapped powdery mildew resistance gene loci are presented in

Figure 1.

The presence ofPm resistant genes is vital not only for monogenic resistance but also the defeated Pm genes often

confer oligogenic and quantitative type resistance when combined together (Royer et al., 1984; Pedersen & Leath,

1988; Paillard et al., 2000). Chromosomal locations, cultivars/lines, sources and references for the 64 known

powdery mildew resistance genes/alleles have been identified as major genes for vertical resistance to powdery

mildew in wheat (Ma et al., 2011; McIntosh et al., 2008; Luo et al., 2009; Li et al., 2009; Hua et al., 2009; He et

al., 2009) (Table 1). Thirty Pm alleles at 25 loci have been nominated for wheat powdery mildew resistance

(McIntosh et al. 1995). Twenty-five alleles at 19 loci from Pm1 toPm19, their locations at chromosomes, and their

sources have been reviewed (McIntosh et al. 1995). OtherPm alleles such asPm20, Pm21, andPm22 have been

reported by Friebe et al., 1994, Qi et al., 1995, and Peusha et al., 1996 andPm25 has been identified by Shi et al.

1998. Host-pathogen interactions analysis, chromosomal (cytogenesis) analysis and molecular marker techniques

have been utilized for determining chromosomal locations ofPm genes. These powdery mildew resistance genes are

non-randomly distributed in the genome (Table 3), but form clusters in gene-rich regions (Gill et al., 1996a, b). The

highest numbers ofPm loci are contained by each 6B and 7B chromosome with 5 knownPm loci. Chromosomes


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without knownPm genes are 3A (Pm44), 4D, and 5A (according to Hsam & Zeller, 2002). Gene loci that contain

more than one resistance allele arePm1 with 4 alleles,Pm3 with 9 alleles,Pm4 with 3 alleles,Pm5 with 4 alleles

(Chen & Chelkowski, 1999; Hsam & Zeller, 2002). Hsam and Zeller (2002) stated that lociPm10,Pm11,Pm14, and

Pm15 contain individual genes for resistance to Erysiphe graminis f.sp. agropyri and are not effective against

Blumeria graminis f. sp. Tritici. Eight gene loci were identified in homoeologous group 1, whereas only one gene

(Pm16) was found in homoeologous group 4 (i.e. chromosome 4A; Reader & Miller, 1991).

Xue et al. (2009) reported that the Chinese landraces wheat line Xiaobaidong contained a new recessive gene, mlxbd

which was located on chromosome 7BL and near the locusPm5. Powdery mildew resistance geneMldwas located

on chromosome 4B in the wheat lines, Halle 13471, H8810/47 and Maris Dove. It was transferred from T. durum

(McIntosh et al., 1995). Zelleret al. (1993b) reported that three wheat cultivars, Abo, Aristide and Courtot, contained

a major gene,Mlar, for resistance to the German Bgtisolate no.2. Robe and Doussinault (1995) reported that the line

RF714 contained a new recessive gene, mlre, for wheat powdery mildew resistance, which was derived from a cross

betweenAegilops squarrosa 33 and Triticum dicoccum 119. They postulated that mlre was derived from T. dicoccum.

A new recessive gene, pmTD1, was identified in the wheat line NC92-8562 transferred from Ae. Tauschii ssp.

Tauschii (Shi et al., 1998). Liu et al. (1989) reported that the variety Kenguia 1 contained a new gene, KG, for

powdery mildew resistance, which was located on chromosome 6A.

3. Sources and Distribution of Resistance Genes

Common sources ofPm genes are different species within the primary, secondary and tertiary gene pools. Breadwheat is an allohexaploid species (2n=6x=42), with three distinct genomes (AABBDD). Many of the resistance

genes were introduced from ancestral and other wild species related to common wheat such as Triticum monococcum,

close relative of the A genome progenitorTriticum uratu, the B genome progenitorAegilops speltoides, and the D

genome progenitorAe. Tauschii (Hsam & Zeller, 2002; Jiang et al., 1994). Chen and Chelkowski (1999) and Hsam

and Zeller (2002) reported a total of 22 resistance alleles at 10 loci includingPm1, Pm2, Pm3 (3a, 3b, 3c, 3d, 3e, and

3f),Pm9, Pm18, Pm22 andPm45 in T. aestivum indicating thatPm genes may still be found in cultivated wheat.

Although Bennett (1984) reported that just a small number ofPm genes have been identified which originated in the

cultivars T. aestivum. Mains (1933) identified that the wild wheat relatives T. monococcum (AA genomes), T.

dicoccum (AABB), and T. timopheevi (AAGG) are the sources of resistance genes to powdery mildew as early as

1933. Screening of old wheat cultivars, land-races and related species for resistance to powdery mildew started in the

1930s (Hsam & Zeller, 2002). Pm genes were identified in many different, widely distributed wheat cultivars and

landraces. Pm5a and Pm5b, followed by Pm2, Pm6, and Pm8 are the most common in Europe, Asia and

Mediterranean cultivars. Pm3a is commonly found in wheat cultivars grown in diverse geographical locations

including the Balkans, Japan, china and the US.Pm3c was identified in Germany, whilePm3dwas found in several

European countries and China.Pm4a has been used in commercial wheat cultivars in Germany and China. A number

of commercially grown cultivars have been found to have Pm gene combinations (Heun & Fischbeck, 1987). The

best known cultivars are Normandie withPm1, Pm2, andPm9, Maris Huntsman withPm2 andPm6, Kronjuvel with

Pm4b andPm8, and 623/65 withPm4b andPm8 (Liu et al., 1999). Gene transfer from species within the primary

gene pool ofTriticum that homologous chromosomes to wheat can be done directly by hybridization, recombination

and backcrossing.

Diploid Aegilops tauschii Coss (2n=2x=14, DD) has proved to be a valuable relative for wheat breeding and

diversifying disease resistance (Gill et al. 1986, Cox et al. 1992). For wheat powdery mildew resistance, Gill et al.

(1986) reported on the reactions of 60 accessions ofA. tauschii to fourBgtisolates. Among the 60 accessions, four

showed an immune reaction, seven were highly resistant, and 20 were moderately resistant. Two resistance alleles,

Pm2 andpm19, were transferred into common wheat fromA. tauschii (Hsam & Zeller, 2002). AlthoughPm10 andPm15 are not effective againstBgt, they can be traced fromA. tauschii (McIntosh et al., 1995). Four new germplasm

lines, NC96BGTD1, NC96BGTD2, were released with wheat powdery mildew resistance alleles, which were

transferred from A. tauschii and two germplasm line NC96BGTD3 and NC97BGTD7 contain Pm34 and Pm35

(Murphy et al., 1998; Mirinda et al., 2006, 2007). Shi et al., (1998) identified new allele(s) for powdery mildew

resistance transferred fromAe. Tauschii ssp. Tauschii in NC92-8562 and NC109-2-1-G1-1; diploid einkorn wheat (T.

monococcum) (2n=2x=14, AA genome) derivative common wheat germplasm NC96BGTA5 contain Pm25 (Shi et

al., 1998; Murphy et al., 1998).

Tetraploid wild emmer wheat (T. dicoccoides) (2n=4x=28, AABB), the progenitor of common tetraploid and

hexaploid wheats (Liu et al., 2002); and the source ofPm16,Pm26,Pm3, Pm31, Pm36, Pm41 and pm42 (Rong et al.,

2000; Liu et al., 2002; Hsam & Zeller, 2002; Hua et al., 2009). Krivchenko et al., (1979) determined the reactions of

29 T. dicoccoides samples to wheat powdery mildew. Twenty-eight were resistant in the field, and fifteen were

resistant in the seedling stage. Wang et al. (2007) reported that the temporary design powdery mildew resistance gene


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PmAS846derived from wild emmer (T. dicoccoides) accession As846. Moseman et al. (1984) reported on the

reactions to powdery mildew of 233 T. dicoccoides nti-infla. Resistance to at least one isolate was found in 149

accessions, and 137 expressed intermediate to complete resistance to fourBgt isolates; tetraploid T. dicoccum

(2n=4x=28, AABB), a source of genes for resistance to powdery mildew (Bennett, 1984; Navarro et al., 2000; Hsam

and Zeller, 2002) includingPm4a and Pm5a ; tetraploid T. durum (2n=4X=28, AABB), a somewhat less valuable

source of resistance to powdery mildew (Mains, 1934; Hsam & Zeller, 2002), although it contributedPm3h (Zeller

and Hsam, 1998); This species is highly regarded as a source forPm and other resistance genes (Mains, 1934;Grechter-Amitai and Van Silfhout, 1984; Hsam & Zeller, 2002); tetraploid T. carthlicum (2n=4x=28, AABB

genomes) was a donor ofPm4b andPm33 (Hsam & Zeller, 2002; Zhu et al., 2005).

Polyploid Triticum andAegilops genotypes sharing at least one common genome with T. aestivum belong to the

secondary gene pool. If genes are on the homologous chromosomes, gene transfer may be by direct hybridization, or

may require special cytogenetic techniques such as embryo rescue (Jiang et al., 1994). Some diploid and tetraploid

species belong to this group and some species have been used as sources of resistance genes such as; tetraploid

cultivated T. timopheevii and its wild form, T. araraticum, (2n=4x=28, AAGG), contributedPm6, Pm27, Pm37and

contain at least one more Pm gene (Mains, 1934; Jrve et al., 2000; Hsam & Zeller, 2002; Murphy et al., 2002;

Perugini et al., 2008);Ae. Speltoides (2n=2x=14, SS) was the donor ofPm1d,Pm12 andPm32 (Hsam and Zeller,

2002; Hsam et al., 2003); and Ae. Longissima (2n=2x=14, SS), was the donor ofPm13 (Cenci et al., 1999). Ae.

Speltoides andAe. Longissima are both diploid species with the S genome, which is closely related to the B genome

of wheat and show co-linearitywith at least five chromosomes with the wheat D genome (Zhang et al., 2001; Hsam& Zeller, 2002). Other species such as Dasypyrum (Hylandia) (2n=2x=14, VV), cultivated rye (Secale nti-i)

(2n=2x=14, RR), and someAegilops species which do not share with common wheat genomes belong to the tertiary

gene pool. A homologous recombination with such donor parents cannot be used for gene transfer. Genetic

techniques such as induction of chromosome translocations radiated or induced mutation at Ph1 locus on

chromosome 5BL or lack of 5B chromosomal pair can be used to facilitate gene transfer (Jiang et al., 1994). The

products of these methods are wheat/alien chromosome translocation, or recombination lines. FourPm (Pm7,Pm8,

Pm17, and Pm20) genes were transferred from rye (Secale nti-i, 2n=14, RR) into cultivated wheat (Hsam and

Zeller, 2002). The 1RS chromosome arm from rye is the most widely incorporated alien chromatin in present wheat

genomes (Hsam et al., 2000). Wheat germplasm Transec contain Pm7as a 4BS.4BL-5RL translocation (Table 1).

Pm8 derived from rye cultivar Petkus (Ren et al., 1997) and Pm17are both located on the short arm of the 1R

chromosome in rye.Pm8 andPm17segregated independently from each other in Amigo wheat which indicated two

distinct translocations.Pm8 is located in T1BL.1RS, andPm17is located in T1AL.1RS wheat-rye translocation lines(Heun et al., 1990; Friebe et al., 1994).Pm20 was transferred from the 6RL rye chromosome into common wheat.

Aegilops nti- (2n=4X=28, UUMM) was the donor ofPm29 and the wild diploidHyanaldia vilosa (2n=2x=14, VV)

was the donor ofPm21 (Zelleret al., 2002). Other species with potentially useful powdery mildew resistance genes

are Ae. nti-in, Ae. Markgrafii, Ae. Umbelluata, Ae. Variabilis, Ae. Triuncalis, and Ae. Mutica, as well as the

perennial subspecies ofTriticae, such asElymus, Leymus, Elytrigia and Thinopyrum, (Jiang et al., 1994; Eser, 1998;

Hsam & Zeller, 2002; Luo et al., 2009).

4. Molecular Markers Linked to Powdery Mildew Resistance Gene

Molecular markers are tools that help to locate and identify parts of DNA that are located near a gene or genes of

interest. DNA markers identify locations where the sequences differ among varieties. These can be located within

genes or in the DNA between genes, so long as they are unique sequences and differ between the plants of interest.

Differences of this type are called polymorphisms, and there are a variety of ways to detect and use these signposts

within the chromosomes (Suslow et al. 2002). Different molecular techniques have been used to characterize andmanipulate resistance genes and to dissect different types of resistance. Molecular markers were used for mapping

monogenic resistance, characterization of quantitative resistance in germplasms and marker-aided selection

(Michelmore, 1995). Molecular identification of specific DNA sequences can be used to identify the presence or

absence ofPm genes in a cultivar, their chromosomal location, the number of genes and the way in which they are

transmitted to progeny (Chen & Chelkowski, 1999). With the help of molecular markers, more than 20 powdery

mildew resistance genes, such asPm30 (Liu et al., 2002),Pm31 (Xie et al., 2003),Pm33 (Zhu et al., 2005),Pm34

(Miranda et al., 2006),Pm35 (Miranda et al., 2007),PmY39 (Zhu et al., 2006),PmY201 andPmY212 (Sun et al.,

2006),PmU(Qiu et al., 2005),MlZec1 (Mohleret al., 2005), Mlm2033,Mlm80 andpm2026(Yao et al., 2007; Xu et

al., 2008),PmLK906(Niu et al., 2008)Pm43 (He et al., 2009) andPm45 (Ma et al., 2011), have been discovered and

mapped. Molecular marker techniques commonly used for identification and confirmation ofPm genes to powdery

mildew are:


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4.1 Restriction Fragment Length Polymorphisms (RFLP)

RFLPs were the first molecular markers that developed and used in genetic analysis, initially in humans (Botstein et

al., 1980), and later applied to plants (Weber & Helentjaris, 1989). Even though these markers were extensively used

for mapping approaches in various plant species, they didnt fulfill the initial expectations as universal genotyping

assays, since they require large amounts of DNA, are expensive and time consuming. Devos & Gale (1993)

considered the RFLP technology too slow and too expensive to use for routine screening of the mapping populations.

Thus, RFLPs have limited application in wheat breeding programs that require large-scale screening of progenies

from intra-specific crosses in a short time period. A large number of identified powdery mildew resistance genes have

been tagged with RFLP markers in wheat, such as:Pm1, Pm2, Pm3b, Pm4a (Ma et al., 1994),Pm1, Pm2, andPm18

(Hartl et al., 1995),Pm1c (Hartl et al., 1999),Pm2 (Mohler & Jahoor, 1996),Pm2, Pm4a, Pm21 (Liu et al., 2000),

Pm3a, Pm3b, andPm3c (Hartl et al., 1993b),Pm3g(Sourdille et al., 1999),Pm6(Tao et al., 2000),Pm12 (Jia et al.,

1996),Pm13 (Cenci et al., 1999), Pm17(Hsam et al., 2000), Pm21 (Liu et al., 1999), Pm26(Rong et al., 2000),

Pm27(Jrve et al., 2000) andPm29 (Zelleret al., 2002).

4.2 Random Amplified Polymorphisms (RAPD)

RAPD is based on the amplification of random DNA segments using a single primer of arbitrary nucleotide sequence.

Sequence information and radioactivity are not required for RAPD analysis. It is economical and easy to use. Several

powdery mildew resistance genes tagged with RAPD markers, such as: Pm1 (Hu et al., 1997), Pm1, Pm2, Pm3,

Pm3a, Pm3b, Pm3c, Pm4a,Pm12 (Shi, 1997),Pm13 (Cenci et al., 1999),Pm18 (Hartl et al., 1995),Pm21 (Qi et al.,1996; Liu et al., 1999), andPm25 (Shi et al., 1998). More than forty RAPD markers and two RFLP markers have

been identified to be associated withPm1, Pm2, Pm3a, Pm3b, Pm3c, Pm4a, andPm21 (Hartl et al., 1993b, Ma et al.,

1994, Qi et al., 1996). However, most RAPD markers are dominant, and sometimes the results are difficult to

reproduce. Fortunately, RAPD markers can be converted to more reliable markers, such as SCARs (sequence

characterized amplified regions). For example, the RAPD marker linked to the wheat powdery mildew resistance

genePm21 was converted to a SCAR marker (Liu et al., 1996).

4.3 Amplified Fragment Length Polymorphisms (AFLP)

The AFLP technique is based on selectively amplifying a subset of restriction fragments from a complex mixture of

DNA fragments obtained after digestion of genomic DNA with restriction endonucleases (Vos et al., 1995). AFLP

analysis is a reliable and efficient method and a powerful technique to generate large numbers of markers for the

construction of high-density genetic maps (Beckeret al., 1995; Keim et al., 1997), identifying specific genes (Kasuga

et al., 1997; Schwarz et al., 1999) and map-based cloning of resistance genes (Buschges et al., 1997; Wei et al.,1999). Linked AFLP markers have already been found forPm1c andPm4a (Hartl et al., 1999),Pm17(Hsam et al.,

2000),Pm24 (Huang et al., 2000b),Pm29 (Zelleret al., 2002) andpm42 (Hua et al., 2009).

4.4 Microsatellites (Simple Sequence Repeat, SSR)

Microsatellites or simple sequence repeats (SSRs) are an alternative type of codominant marker more suitable for

screening large populations than RFLPs. They are simple sequence repeats of only a few base pairs (1-6) that are

commonly found in eukaryotic genomes (Gupta et al., 1999). Most of microsatellite markers are chromosome

specific, thereby simplifying the assignment of linkage groups (Rderet al., 1998; Gupta et al., 1999). The genome

specificity of microsatellite markers can also be used to recognize the arm and sub-arm localization of disease

resistance genes using Chinese Spring ditelosomic and deletion stocks (Endo & Gill 1996). Gene-flanking

microsatellite markers can be assigned to chromosome arms and interval breakpoints by examining their presence or

absence in ditelosomic and deletion lines (Plaschke et al., 1996; Sourdille et al., 2004). Over 1000 microsatellites

from wheat are currently available (Gupta et al., 2002 ; Guyomarch et al., 2002 ; Huang et al., 2003b ; Rderet al.,2004 ; Song et al., 2002 ; Stephenson et al., 1998). Recently, using SSR markers, there are several powdery mildew

resistance genes identified and mapped such as :Pm24 (Huang et al., 2000b),Pm27(Jrve et al., 2000),Pm30 (Liu

et al., 2002),Pm33 (Zhu et al., 2005),Pm34 (Miranda et al., 2006),Pm35 (Miranda et al., 2007), Pm36(Blanco et

al., 2008) Pm40 (Luo et al., 2009), Pm41 (Li et al., 2009) pm42 (Hua et al., 2009), Pm43 (He et al., 2009) and

Pm45(Ma et al., 2011).

4.5 Sequence Tagged Site (STS)

STS markers are single copy sequence amplified using specific primers that match the nucleotide sequences at the

ends of a DNA fragment of an RFLP probe (Olson et al., 1989). This approach is extremely useful for studying the

relationship between various species and linked to some specific traits (Bustos et al., 1999; Hartl et al., 1993a). RFLP

probes specifically linked to a desired trait can be converted into PCR-based STS markers, based on nucleotide

sequence of the probe giving polymorphic band pattern, to obtain specific amplification. Tedious hybridization


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procedures involved in RFLP analysis can be overcome using this technique. Tagged STS markers have been

identified forPm1 (Hu et al., 1997),Pm2 (Mohler & Jahoor, 1996),Pm13 (Cenci et al., 1999),Pm41 (Li et al., 2009)

and pm42 (Hua et al., 2009).

5. Mapping Population

Both F2 and backcross populations are easy to construct and can be produced within short time. F 2 is more

powerful for detecting QTLs with additive effects, and can also be used to estimate the degree of dominance fordetected QTLs. When dominance is present, backcrosses give biased estimates of the effects because additive

and dominant effects are completely irritating in this design (Carbonell et al., 1993). Many markers require to be

analyzed for a large number of plants when F2 or backcross populations are used for gene mapping. Besides, some

traits are difficult to score on an individual plant basis. So, alternative strategies have been used to improve the

efficiency of genetic mapping such as NILs (near-isogenic lines), BSA (bulked segregant analysis) and RILs

(recombinant inbred Lines) lines or DH (double haploid, Michelmore et al., 1991).

NILs that differ by the presence or absence of the target gene and flanking a small region of DNA, are useful to

identify markers linked with the target gene (Young et al., 1988). Genetic markers are polymorphic between the NIL

and its recurrent parent that are putatively linked to the target gene (Muehlbauer et al., 1988). Many disease

resistance genes have been mapped using NILs, including powdery mildew resistance in wheat and barley (Hinze et

al., 1991; Schulleret al., 1992).Pm2, Pm3, Pm4a andPm6have been mapped using NILs (Hartl et al., 1995; Tao et

al., 2000).Although NILs are helpful to construct gene map, often they are unavailable, and the development of NILs is

time-consuming and laborious. To overcome the problems of NILs, Michelmore et al., (1991) successfully used

bulked segregant analysis (BSA) to identify RAPD markers tightly linked to genes for resistance to lettuce downy

mildew. Many powdery mildew resistance gene/allele such as Pm1, Pm4a, Pm8, Pm24, Pm25, Pm29, Pm30 and

Pm31 have been identified using BSA (Shi et al., 1998). This strategy involves comparing two DNA samples pool of

individuals from a segregating population. Within each pool, or bulk, the individuals are identical for the trait or gene

of interest but are uninformed for all the other genes. All polymorphic markers between two DNA pools are

putatively linked with the target gene.

Recombinant inbred lines or double haploid populations are permanent populations that can be used indefinitely for

mapping. They can also be readily disseminated among labs and new data can be continuously added to a

pre-existing map. Furthermore, RI lines or DH populations can be evaluated in many different environments. Since

each genotype is represented by an inbred line, rather than by an individual plant, a more accurate assessment of thegenetic component of variance can be made in studying quantitative traits (Burret al., 1988). Therefore, RI lines or

DH populations are more useful for analysis of quantitative traits or traits that are difficult to characterize on an

individual plant basis. DH lines have been used to screen molecular markers associated with genes,Pm3a,Pm3gand

Pm8 for powdery mildew resistance in wheat (Hartle et al., 1993b; Sourdille et al., 1999; Wricke et al., 1996).Pm13

has been mapped using RI lines (Donini et al., 1995)

6. Mapping of Powdery Mildew Resistance Genes

The development of genetic maps of wheat is now adding a new dimension for identification of molecular markers

associated with powdery mildew resistance genes. Screening markers can be conducted in the two parents, by

selecting several markers on each chromosome of the genetic map, and then linkage between the allele for resistance

and the polymorphic markers in the two parents can be estimated by use of QTL statistical analysis based on the data

from a segregating population. In plants, molecular mapping and cloning of disease resistance genes will facilitate

the study of molecular mechanisms underlying and evolution of resistance and will permit marker-assisted selectionin breeding programs. Several powdery mildew resistance genes have been tagged with molecular markers (Table 2).

Using cultivar Chancellor as the recurrent parent, Briggle (1969) developed NILs for powdery mildew resistance

genesPm1, Pm2, Pm3 andPm4a, respectively. Hartl et al. (1995) found that RFLP marker Whs178 was 3 cM away

from gene Pm1. Hu et al. (1997) used RAPD markers to tag gene Pm1. RAPD markers UBC320420 and

UBC638550 cosegregated with genePm1 among 244 F2plants. Another RAPD marker OPF12650 was 5.4 cM away

from gene Pm1. Recently, Hartl et al. (1999) have used AFLP markers to map gene Pm1c. Among 96 primer

combinations, 31 polymorphic AFLP fragments between the resistant and susceptible pools were in accordance with

the patterns of the parents. The eight most reliable polymorphic markers were analyzed in a segregating population

for the genePm1c. Two of them cosegregated with the genePm1c and the other six markers were tightly linked with

the gene. One AFLP marker, 18M2, was found to be highly specific for the Pm1c gene in diverse genetic

backgrounds. RFLP analysis of NILs possessing the gene Pm2 and the recurrent parent indicated that: 1) RFLP

marker BCD1871 was 3.5 cM away from gene Pm2 (Ma et al., 1994); 2) RFLP marker Whs295 mapped 2.7 cM


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away from the genePm2 (Hartl et al., 1995); and 3) the genePm2 was also linked with RFLP marker Whs350 (Hartl

et al., 1995). Ma et al., (2011) found that Pm45 on chromosome 5DS which was flanked by Xgwm205 and

Xmag6176, with a genetic distance of 8.3 cM and 2.8 cM, respectively. This gene was 3.3 cM from a locus mapped

by the STS marker MAG6137, converted from the RFLP marker BCD1871, which was 3.5 cM fromPm2. Using

RFLP analysis of NILs possessing the gene Pm3 and the recurrent parent, Hartl et al. (1993b) found that RFLP

marker Whs179 revealed polymorphism not only between the NILs with and without gene Pm3, but also among

NILs possessing different alleles of the Pm3 locus. The genetic distance between probe Whs179 and Pm3 was3.31.9 cM. Ma et al. (1994) reported that RFLP marker BCD1434 was 1.3 cM away fromPm3a orPm3b. Ma et al.

(1994) also reported that Pm4a cosegregated with RFLP markers BCD1231-2A(2) and CDO678-2A, and was

closely flanked by BCD1231-2A(1) and BCD292-2A. Xue et al. (2009) reported that SSR marker Xgwm577 was

linked to powdery mildew resistance gene mlxbdwith a distance of 3.5cM. Blanco et al., (2008) found that Pm36

linked on chromosome 5BL with five AFLP markers XP43M32 (250), XP46M31(410), XP41M37(100),

XP41M39(250) and XP39M32(120), three genomic SSR markers (Xcfd07, Xwmc75, Xgwm408) and one

EST-derived SSR marker (BJ261635). Using F2 population Zhang et al. (2010) also found that the temporary design

powdery mildew resistance geneMl3D232 on chromosome 5BL which was flanked by Xgwm415 and Xwmc75.

Zhang et al. (2009) also found temporary Pm design gene MIW29 on 5BL chromosome and also flanked by

Xgwm415 and Xwmc75, with a genetic distance of 2.5 cM and 17.6 cM, respectively.

7. Conclusion

Molecular markers tightly linked to economically important monogenic or oligogenic trait have potential for

immediate utility in plant improvement. Efficient application of molecular markers in plant breeding will depend on

the development cost-effective and automated diagnostic technologies. A major problem is when the linked marker

used for selection is at a distance away from the gene of interest, leading to crossover between the marker and the

gene. In future, the success of marker assisted selection may depend on the possibility of tagging the favorable alleles

themselves.

Valuable lessons learnt from past research are likely to encourage more researchers to develop reliable markers and

plant breeders to adopt MAS. PCR-based markers are more attractive for MAS, due to the small amount of template

required and more efficient handling of large population sizes. PCR-based molecular markers are suitable for marker

assisted selection (MAS), due to small amount of DNA require, more efficient managing of large population sizes

and possible to map and tag almost any trait. DNA markers have facilitated the dissection of the genetic basis of

complex traits and have helped in understanding their mode of action and how their functioning is modulated by the

environment. AFLP, RAPD and STS markers can not be applied for differentiation of homozygous and heterozygous

individuals in segregating population. Among the DNA marker systems of wheat, microsatellites are recently the

optimal marker for MAS, because of their co-dominant inheritance, chromosome-specific and evenly distributed

along chromosomes. A large numbers of microsatellite makers are available that offer identification and molecular

mapping of powdery mildew resistance gene in wheat (Gupta et al., 2002; Guyomarch et al., 2002; Huang et al.,

2001; Roderet al., 2004; Song et al., 2002; Stephenson et al., 1998). Already somePm genes have been identified

and mapped by specific nti-inflammat markers.

We believe that several other factors will greatly affect the efficiency and effectiveness of linkage mapping and MAS

research in the future: new developments and improvements in marker technology, the integration of functional

genomics with linkage mapping, and the availability of more high-density maps.

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Table 1. Chromosomal location, cultivar/line, source and Reference of identified powdery mildew resistance

genes

Gene Location Cultivar/line Source Reference

Pm1a 7AL Axminister T. aestivum Hsam et al., 1998

Pm1b 7AL MocZlatka T. monococcum Hsam et al., 1998

Pm1c(Pm18) 7AL Weihestephan M1N T. aestivum Hsam et al., 1998

Pm1d 7ALT. spelta var.

duhamelianumT. spelta Hsam et al., 1998

Pm1e

(Pm22)7AL Virest T. aestivum Singrun et al., 2003

Pm2 5DS Ulka/XX 194 T. aestivum/Ae. Tauschii

McIntosh & Baker,

1970 and Lutz et

al.,1995

Pm3a 1AS Asosan Briggle & Sears, 1966

Pm3b 1AS Chul T. aestivum Briggle,1966

Pm3c 1AS Sonora T. aestivum Briggle, 1966

Pm3d 1AS Kolibri T. aestivum Zelleret al.,1993a

Pm3e 1AS W150 T. aestivum Zelleret al.,1993a

Pm3f 1AS Michigan Amber T. aestivum Zelleret al.,1993a

Pm3g 1AS Aristide T. aestivum Zeller & Hsam, 1998

Pm3h 1AS Abessi T. durum Zeller & Hsam,1998

Pm3i 1AS N324 T. aestivum Zeller & Hsam, 1998

Pm3j 1AS GUS 122 T. aestivum Zeller & Hsam, 1998

Pm4a 2AL Khapli T. dicoccum The et al., 1979

Pm4b 2AL Armada T. carthlicum The et al., 1979

Pm4d 2AL Tm27d2 T. monococcum Schmolke et al., 2011

Pm5a 7BL Hope T. dicoccum Law & Wolfe, 1966

Pm5b 7BL Ibis T. aestivum Hsam et al., 2001

Pm5c 7BL KolandiT. aestivum ssp.

SphaerococcumHsam et al., 2001

Pm5d 7BL IGV 1-455 T. aestivum Hsam et al., 2001

Pm5e 7BL Fuzhuang 30 T. aestivum Huang et al. 2003a

mlxbd(Pm5) 7BL Xiaobaidong T. aestivum Huang et al., 2000a

Pm6 2BL TP 114 T. timopheevii Jrgensen,1973

Pm7 4BS.4BL-2RL Transec S.cereale Friebe et al., 1994

Pm8 1RS.1BL Disponent S.cereale Hsam & Zeller, 1997

Pm9 7AL N14 T. aestivum Hsam et al., 1998

Pm10 1D Norin 26 T. aestivum Tosa et al., 1987

Pm11 6BS Chinese Spring T. aestivum Tosa et al., 1988

Pm12 6BS-6SS.6SL Trans.line 31 Ae.speltoides Jia et al., 1996

Pm133BL.3SS-3S

3DL.3SS-3SCstrans.line Ae.longissima Ceoloni et al., 1992


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Gene Location Cultivar/line Source Reference

Pm14 6BS Norin 10 T. aestivum Tosa & Sakai, 1990

Pm15 7DS Norin 26 T. aestivum Tosa & Sakai, 1990

Pm16 4A Norman rec. line T. dicoccoides Reader & Miller, 1991

Pm17 1RS.1AL Amigo S.cereale Heun et al., 1990Pm19 7D XX 186 Ae.tauschii Lutz et al., 1995

Pm20 6BS.6RL KS93WGRC28 S.cereale Friebe et al., 1994

Pm21 6VS.6AL Yangmai 5 line Haynaldia villosa Chen et al., 1995

Pm23(Pm4c) 2AL 82-7241 T. aestivum McIntosh et al., 1998

Pm24 1DS Chiyacao T. aestivum Huang et al., 2000b

Pm25 1A NC96BGTA5 T. boeoticum Shi et al., 1998

Pm26 2BS TTD140 T. dicoccoides Rong et al., 2000

Pm27 6B-6G 146-155-T T. timopheevii Jarve et al., 2000

Pm28 1B Meri T. aestivum Peusha et al., 2000Pm29 7DL Pova A. ovata Zelleret al., 2002

Pm30 5BS C20 T. dicoccoides Liu et al., 2002

Pm31(MIG)

cancel6AL G-305-M/781//Jing4113 T. dicoccoides Xie et al., 2003

Pm32 1BL.1SS L501 Ae. Speltoides Hsam et al., 2003

Pm33 2BL PS5 T. carthlicum Zhu et al., 2005

Pm34 5DL NC97BGTD7 Ae. Tauschii Miranda et al., 2006

Pm35 5DL NC96BGTD3 Ae. Tauschii Miranda et al., 2007

Pm36 5BL MG29896 T. dicoccoides Blanco et al., 2008Pm37 7AL NC99BGTAG11 T. timopheevii Perugini et al., 2008

Pm38 7DS RL6058 T. aestivumSpielmeyeret al.,

2005

Pm39 1BL Saar T. aestivum Lillemo et al., 2008

Pm40 7BS GRY19 Elytrigia intermedium Luo et al., 2009

Pm41 3BL IW2 T. dicoccoides Li et al., 2009

pm42 2BS G-303-1M T. dicoccoides Hua et al., 2009

Pm43 2DL CH5025 Thinopyrum intermedium He et al., 2009

Pm44 3AS Hombar T. aestivum Chen et al., 2011

Pm45 6DS D57 T. aestivum Ma et al., 2011


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Table 2. Molecular markers linked to major powdery mildew resistance genes

Gene LocationType of

markersClosest/flanking marker

Linkage

distance/

contribution

Mapping

populationReference

Pm1a 7AL RAPD, STS UBC320420, UBC638550Both

co-segregate

F5, F2 lines,

BSA

Hu et al., 1997

RFLP WHS178-9.4kb-EcoRI 2.8 2.7cM F2 lines,NILs Hartl et al., 1995

RFLP CDO347 Co-segregate F2 lines, NILs Ma et al., 1994

RFLP, STS

mwg2062, cdo347, psr121,

psr148, psr680, psr687,

wir148, C607, STS638542,

ksuh9

All

Co-segregateF2 lines Neu et al., 2002

Pm1c 7AL RFLP, RAPDWHS178-15kb-EcoRI,

OPH-111900

4.4 3.6cM,

13cMF2 lines, BSA Hartl et al., 1995

AFLP S19M22-325/200 Co-segregateF3 + F4 lines,

BSAHartl et al., 1999

S14M20-137/138 Co-segregate

Pm1e 7AL SSR, AFLP GWM344-nullS13M26-372 0.9cM,0.2 cM F2:3 lines, BSASingrun et al.,

2003

Pm2 5DS RFLPWHS350-6.5kb-EcoRV,WHS2

95

3.8cM,2.72.6

cMF2 lines, NILs Hartl et al., 1995

RFLP BCD1871 3.5cM F2 lines, NILs Ma et al., 1994

STS STSwhs350 - F2 lines, NILsMohler &

Jahoor,1996

Pm3a 1AS RFLP WHS179 3.3 1.9cM DH, NILs Hartl et al., 1993b

Pm3b 1AS RFLP BCD1434 1.3cM F2 lines, NILs Ma et al., 1994

Pm3g 1AS RFLP Gli-A5 5.2cM DHSourdille et al.,

1999

Pm4a 2AL RFLP BCD1231, CDO678 Co-segregate, F2 lines, NILs Ma et al., 1994

AFLP 4aM1 3.5cMF3 + F4 lines,

BSAHartl et al., 1999

STS STSbcd1231-1.7kb Co-segregate NILs Liu et al., 1998

Pm5e 7BL SSR GWM1267-136 6.6cM F2:3 lines, BSAHuang et al.,

2003a

Pm6 2BL RFLP BCD135-9kb-EcoRV 1.6 1.5cM F2 lines, NILs Tao et al., 2000

Pm8 1RS.1BL RFLP IAG95 Tightlylinkage F2 lines, BSA Wricke et al.,1996RAPD OPJ07-1200, OPR19-1350 -

Translocation

lines

Iqbal & Rayburn,

1995

STS SEC-1b-412bp -Translocation

lines

deFroidmont,

1998

STS STSiag95-1050 Co-segregate DH, F2:3 linesMohleret al.,

2001

Pm126BS-6SS

.6SLRFLP

psr10, psr106, Nor-2, psr141,

psr113, psr142, psr149, psr2Co-segregate F2 lines Jia et al., 1996

Pm13 3BL.3SS RFLP psr305, psr1196 - Recombinant Donini et al., 1995


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ISSN 1925-430X E-ISSN 1925-431838

Gene LocationType of

markersClosest/flanking marker

Linkage

distance/

contribution

Mapping

populationReference

-3S

3DL.3SS

-3S

lines

RFLP, RAPD,

STS

cdo460, utv135, OPV13800,

UTV13, OPX12570, UTV14-

Recombinant

lines Cenci et al., 1999

Pm171RS.1A

LRFLP, AFLP IAG95CA/CT-355 1.5cM F2:3 lines Hsam et al., 2000

Pm216VS.6A

LRAPD OPH171900 Co-segregate F2 lines Qi et al., 1996

RAPD, SCAROPH171400, SCAR1265,

SCAR1400

Allco-segregate

F2 lines Liu et al., 1999

Pm24 1DS AFLP, SSR E34/M51-407, Xgwm337-204 Co-segregate,2.4 1.2cM

F2:3 lines, BSA Huang et al.,2000b

SSR Xgwm1291 Co-segregate F2:3 linesHuang & Roder,

2003

Pm25 1A RAPD OPA04950 12.8cMBC1F1 lines,

BSAShi et al., 1998

Pm26 2BS RFLP wg516 Co-segregate RSLs Rong et al., 2000

Pm27 6B-6G RFLP, SSR psp3131 Co-segregate F2 lines Jarve et al., 2000

Pm29 7DL RFLP, AFLP

S24M13-233, S19M23-240,

S22M26-192, S25M15-145,

S13M23-442, S22M21-217,S17M25-226

Allco-segregat

e

F2 lines, BSA Zelleret al., 2002

Pm30 5BS SSRXgwm159-460,

Xgwm159-50056cM

BC2F2 lines,

BSALiu et al., 2002

Pm31 6AL SSR Xpsp3029 0.6cMBC2F2 lines,

BSAXie et al., 2003

Pm36 5BL SSR BJ261635 Co-segregate BC5F5Blanco et al.,

2008

Pm40 7BS SSR Xgwm297 0.4cM F2 lines Luo et al., 2009

Pm41 3BLSSR, ISBP,

STS

BE489472 Co-segregate F2 lines Li et al. (2009)

Pm42 2BS

SSR,

FLP-SCAR,

EST-STS,

RFLP-STS

BF146221 Co-segregate F2 lines Hua et al., 2009

Pm43 2DL SSR Xwmc41 2.3cMF3 and BC1

linesHe et al., 2009

Pm45 6DS SSR, STS Xmag6176 2.8cM F2 lines Ma et al., 2011


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Table 3. Distribution of powdery mildew resistance genes among homoelogous chromosomes in wheat and Rye

Homoelogous

groupA B D R

1 Pm3, Pm25 Pm28, Pm32 Pm10, Pm24 Pm8, Pm17

2 Pm4, Pm23 Pm6, Pm26, Pm33, pm42 Pm43 Pm7

3 Pm44 Pm13, Pm38, Pm41

4 Pm16

5 Pm36, Pm16,Pm30 Pm2, Pm34, Pm35

6 Pm21, Pm31Pm11, Pm12, Pm14,

Pm27Pm45 Pm20

7Pm1, Pm9, Pm18,

Pm37Pm5, Pm40 Pm15, Pm19, Pm29, Pm39

2DL 3BL

2BS

6DS 7BS 5BL

Figure 1. Genetic linkage maps of powdery mildew resistance gene-Pm43 (He et al., 2009),Pm41 (Li et al. 2009),

pm42 (Hua et al., 2009),Pm45 (Ma et al., 2011),Pm40 (Luo et al., 2009) andPm36(Blanco et al., 2008)

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Powdery Mildew Resistance Genes in Wheat: Identification and Genetic Analysis